1. Field of the Invention
The present invention relates to a lithium battery, and more particularly, to a method of preparing a lithium secondary battery comprising a multi-phase separator and a lithium secondary battery made thereby.
2. Description of the Related Art
With the rapidly continuing development of miniaturized, lightweight and wireless electronic devices, such as cell phones, camcorders, laptop computers and the like, high-energy density lithium secondary batteries are the focus of an intense investigation for use as power sources for driving such electronic devices. According to the type of electrolyte used, a lithium secondary battery can be classified as a lithium ion battery, which uses a liquid electrolyte, or a lithium ion polymer battery, which uses a solid electrolyte. The lithium ion polymer battery using a solid electrolyte is less prone to leakage and has excellent processibility for use as a battery pack.
A gel polymer electrolyte or a hybrid polymer electrolyte is generally used as the polymer electrolyte in the lithium ion battery. The gel polymer electrolyte battery refers to a battery fabricated by solidifying a solution containing a large amount of solvent and/or a plasticizer, a lithium salt and a polymer electrolyte. Since the gel polymer electrolyte battery has poor mechanical strength, the thickness of an electrolyte layer is generally 75 μm or greater, which is too thick compared to a lithium ion battery, resulting in a reduction of energy density. In order to overcome this drawback, there is a known technology in which a porous separator having good mechanical strength is used and the separator is treated with a gelled polymer capable of absorbing an electrolyte (specifically a lithium salt).
In another conventionally proposed technology, a polymer electrolyte is coated on an electrode using an auxiliary solvent for fabrication of a battery. However, according to this technology, it is necessary to strictly control moisture during the process, and separate coating steps are required for a cathode and an anode.
A further conventional technology includes coating a gelled polymer containing a salt onto a porous separator. However, this technology also has a disadvantage in that moisture should be strictly controlled.
Also, there is a known technology in which a gelled polymer is coated onto a porous separator and an electrolytic solution is injected into the porous separator to then be solidified. However, this method has several drawbacks. For instance, a time required for uniform impregnation of the electrolytic solution during injection of the electrolytic solution is prolonged. Also, in the case of a stacking-type battery in which electrode plates should be tightly bonded to a separator, it is quite difficult to tightly bond the electrode plates under the temperature and pressure conditions where micropores of the porous separator are not structurally deformed.
Further, another conventional technology includes fabricating a battery such that a cathode, an anode and a separator are stacked using a binary solvent of EC and PC as a plasticizer. However, when the binary solvent is used for a cathode or an anode, it is difficult to uniformly inject an electrolytic solution into the electrodes during the process, and mechanical strength is weakened, resulting in internal shorting.